In the measurement of accelerated corrosion rate as disclosed in my U.S. Pat. No. 3,694,324, the corrosion current i.sub.A occurring at the free electrode potential is measured first.
This measurement of current i.sub.A, when made according to my U.S. Pat. No. 3,250,689, employs a corrosion cell containing the ionically conducting corrosive into which are immersed a measured electrode, a reference electrode for measuring its potential, and an opposed electrode for passing DC current to or from the measured electrode. A DC voltage is applied to the measured and opposed electrodes to polarize the measured electrode within the substantially linear relationship range between polarizing DC current and resulting polarization voltage that extends from slightly above zero to about 0.03 volt of polarization in either anodic or cathodic direction, and measurement is made of the polarizing current i.sub.p and of the resulting polarization voltage e.sub.p after the current-potential relationship first approaches a selected slow rate of change. Then, according to my U.S. Pat. No. 3,156,631, the measured current i.sub.p is converted to the corrosion current i.sub.A through linear proportionality with the Direct Voltage E.sub.d = 0.028 to 0,030 volt, as i.sub.A = i.sub.p (1/2E.sub.d /e.sub.p). The polarization voltage e.sub.p may be produced in cathodic direction in the form of e.sub.pc, or in the anodic direction in the form of e.sub.pa, and the corrosion current i.sub.A can be measured from the average of these two measurements.
When current i.sub.A is measured according to my U.S. Pat. No. 3,069,332, only two measured electrodes are required, usually in the form of duplicated electrodes. The polarizing DC voltage is applied to these electrodes to produce the polarizing current i.sub.p which polarizes the one electrode cathodically by the voltage e.sub.pc and the other electrode anodically by the voltage e.sub.pa. When the ionic conductor resistance is negligible, the DC voltage applied to the electrodes measures the sum of e.sub.pc and e.sub.pa, and i.sub.A = (i.sub.p)(E.sub.d)/(e.sub.pa +e.sub.pc) = (i.sub.p)(E.sub.d)/(applied DC Voltage). A second measurement of i.sub.A can be made with reversed polarity of the applied DC voltage, to average the two measurements.
Following this measurement of current i.sub.A, the presence or absence of accelerated corrosion is detected by applying an increment of cathodic polarizing current i.sub.x to the measured electrode, or to each of duplicated measured electrodes, through a means not interfering with corrosion current measurement, while measuring polarizing current i.sub.p. If current increment i.sub.x produces an increase in current i.sub.p, there is no evidence of accelerated corrosion mechanism operation, and the rate-determining corrosion current i.sub.R is determined as, i.sub.R = i.sub.A, where i.sub.R can be applied directly through Faraday's Law of Electrolysis to convert current into rate of metal weight loss.
If current increment i.sub.x produces a decrease in current i.sub.p, the presence of accelerated corrosion mechanism is indicated. Measurement is then made of change in value of current i.sub.p produced by increase in value of current i.sub.x, to determine the minimum value of current i.sub.p, termed i.sub.pb, produced by the application of current i.sub.x. Current i.sub.pb is converted to bounding corrosion current i.sub.B through the Direct Voltage E.sub.d, as described above. The accelerated corrosion mechanism then measures corrosion rate as, i.sub.R = 2.4(i.sub.A) - i.sub.B. Alternatively, but generally with less accuracy, the current i.sub.xb at which i.sub.pb occurs, can be measured and taken as i.sub.R = i.sub.xb.
One alternative means for cathodically polarizing the measured electrode by current i.sub.x while not interfering with the corrosion current measurement, is through what is termed a circuit isolation device, operated as follows. When current i.sub.A is measured with two electrodes, they are taken in the form of duplicated measured electrodes. When current i.sub.A is measured with one measured electrode, a reference electrode and an opposed electrode, the reference electrode is taken in the form of a duplication of the measured electrode. The positive pole of a source of variable DC voltage is connected to an additional electrode operated as an anode. The negative pole is connected to two isolation resistors of equal ohmic value. Each of the two duplicated electrodes is connected to the negative pole through one of the isolation resistors. The ohmic value of the isolation resistors is selected to be large enough to cause an acceptable small current to flow through them from DC voltage subsequently applied or produced between the duplicated electrodes during i.sub.p measurement. This current through the isolation resistors may be taken as a maximum value of about 10 % of measured corrosion current i.sub.A. A meter in series with said source of variable DC voltage measures the total cathodic polarizing current 2i.sub.x.
Said U.S. Pat. No. 3,694,324 describes two alternative methods for operating the circuit isolation device. In one alternative, the relationship between cathodic polarizing current i.sub.x and polarizing current i.sub.p of corrosion current measurement is measured from a series of points of relationship measurement made at selected increments of i.sub.x. In the other alternative, the relationship is continuously measured as current i.sub.x is applied at a selected rate of increase. The measurement of the current i.sub.x and i.sub.p relationship from pointwise measurement has one advantage. The increment of current i.sub.x is applied first, and if a difference in potential is produced between the duplicated electrodes, it can be equalized before application of the DC voltage to produce the current i.sub.p measurement. Its disadvantages include the requirement for a plurality of repeated method operation steps which consume time and are not readily automated.
In theory, measurement of the current i.sub.x and i.sub.p relationship through continuous application of current i.sub.x at a selected rate of increase minimizes the time required for measurement of current i.sub.pb or i.sub.xb, and is more easily automated. In practice, such continuous measurement if found to be unreliable even under the most favorable conditions of laboratory precision in using duplicated electrodes of equal area and isolation resistors of equal ohmic value. Its reliability is further decreased when the anode electrode of the circuit isolation device is brought to close equidistant spacing from the duplicated electrodes, as is required for a corrosion probe of reasonably small diameter. To illustrate this, in some instances initial increase of i.sub.x causes initial increase of i.sub.p, indicating no acceleration, but further increase of i.sub.x causes a decrease in i.sub.p, and the minimum value, i.sub.pb, then has a large positive error. In other instances, initial increase of i.sub.x causes initial decrease of i.sub.p, indicating acceleration, but further increase of i.sub.x causes i.sub.p to decrease through zero so that i.sub.pb cannot be measured.